1. Field of the Invention
The present invention relates to a temporary adhesive for wafer processing, a member for wafer processing using the same, a wafer processed body, and a method for producing a thin wafer.
2. Description of the Related Art
Three-dimensional semiconductor mounting has become an essential process for achieving higher densities and higher capacities. The three-dimensional mounting technique is a semiconductor fabrication technique in which single semiconductor chips are reduced in thickness, and a plurality of layers of these chips are then stacked together with through-silicon vias (TSV) used for electrical connections between the layers. In order to realize this type of structure, a substrate having a semiconductor circuit must be subjected to grinding of the non-circuit-forming face (also referred to as the “back surface”) to reduce the thickness of the substrate, and electrodes including TSVs must then be formed on the back surface. Heretofore, during the back surface grinding of the silicon substrate, a back surface-protective tape is bonded to the opposite side of the substrate to the grinding surface. Thus, wafer breakage is prevented during grinding. However, an organic resin film is used in this tape as the base material. Therefore, the tape has good flexibility, but inadequate strength and heat resistance. The tape is not suited to a TSV formation process and back surface-wiring layer formation process.
Accordingly, a system has been proposed in which the semiconductor substrate is bonded to a support of silicon, glass, or the like, with an adhesive, to achieve a structure that is able to sufficiently withstand the back surface grinding and TSV and back surface electrode formation processes. In this system, an adhesive layer used when bonding the substrate to the support is important. The adhesive must be capable of bonding the substrate to the support with no voids therebetween, have sufficient durability to withstand the subsequent processes, and finally allow the thin wafer to be easily separated from the support. Since this separation is performed in the final process, the adhesive layer is referred to as temporary adhesive layer herein.
As conventionally known temporary adhesive layers and separation methods, techniques have been proposed. One is a technique in which an adhesive containing a light-absorbing substance is irradiated with high-intensity light to decompose an adhesive layer and separate the adhesive layer from a support (Patent Document 1), and the other is a technique in which a heat-meltable hydrocarbon-based compound is used as the adhesive, and bonding and separation are performed with the adhesive in a heated and melted state (Patent Document 2). The former technique requires an expensive apparatus such as a laser, and also suffers from the problem that the processing time per substrate is long. The latter technique is simple since it is controlled only by heating. However, the heat stability at high temperature higher than 200° C. is insufficient. Therefore, the applicable range is narrow. Further, the temporary adhesive layers are not applied to formation of uniform film on a substrate having a high step and complete adhesion to a support.
Further, a technique in which a silicone pressure sensitive adhesive is used for a temporary adhesive layer has been proposed (Patent Document 3). In this technique, a substrate is bonded to a support with an addition-curable silicone pressure sensitive adhesive, and at the time of separation, the bonded substrate is immersed in a chemical agent that dissolves or decomposes a silicone resin and the substrate is detached from the support. This technique requires an extremely extended time for separation, so that application of the technique to an actual production process is difficult.
Since the temporary adhesives are brought to be unnecessary after separation, they are required to be cleaned and removed by an organic solvent, and the adhesives have to be easily dissolved in a cleaning organic solvent. However, the temporary adhesive which is easily dissolved in an organic solvent has a low adhesion force. This may cause problems such as occurrence of a void and separation from a support in a step of processing a substrate. On the other hand, a temporary adhesive having a high adhesion force is unlikely to be separated from a support. In addition, a temporary adhesive layer after the separation is unlikely to be cleaned. Thus, the temporary adhesive has problems such as control of separation force and cleaning property.